The majority of structures used today for the foregoing applications are extremely primitive and take the form of stave bearings generally of wood (lignum vitae) although there are other materials now in use including rubbers, phenolics, etc. Many operators having employed these new materials have however shifted back to lignum vitae because they did not justify the increase in costs without a proportionate decrease in rate of wear down. The lack of availability of the materials in an emergency was also significant.
To describe the water lubricated stern tube stave bearing as a bearing is a technical misnomer. It is merely a guide which grinds itself to destruction and has to be replaced. This is because the separate staves destroy the hydrodynamic film so that hydrodynamic lubrication is impossible at any shaft speed. Wear down is therefore inevitable and sometimes fairly rapid. See "Report of Ships Operators Experience with Stern Tube Bearing Wear". Technical and Research Bulletin No. 3 - 12 published by the Society of Naval Architecs and Marine Engineers, 74 Trinity Place, New York 6, N.Y., in September, 1962.
Apart from the wear of the bearing material, undesirable conditions accompany the wear due to increased vibration caused by shaft whirl, which not only makes the vessel uncomfortable to inhabit, but can lead to destructive effects on the ship's structure.
The result of the use of these inadequate bearings and materials, albeit they are the best known to the industry, is expense in dry docking fees and outage time, frequently leading to replacement of the bearings more often than absolutely necessary because of the uncertainty of wear down rate and the excessive expenses involved in extra drydocking between normal routine overhaul of the ship. It is interesting that in spite of the knowledge of the drawbacks of current bearings, no satisfactory solution has been found.
It is important to realize that because no shaft is ever in precisely correct alignment, because the ship's structure is flexible, and because the shaft alignment can alter radically between partial and full cargo conditions, effects on a fixed bearing can be disastrous.
For the purpose of illustration, let us take the case of a contemporary super-tanker built to the requirements of Lloyds of London. Assume it has a 3 foot tail shaft, and a propeller plus tail shaft weight of say, 60 tons. Lloyds would require a stern tube bearing four diameters long, or 12 feet. Unfortunately this long bearing is of little use, because the overhung mass of the propeller bends the shaft, so that in all of the 12 feet the journal is probably bearing hard at one section only, and because of the resulting inaccuracies proper support of the shaft by hydrodynamic film is impossible. Thus, from its birth the bearing is on its way to its demise. A bearing to be a bearing must be extremely accurate. The bearing just described in extremely inaccurate even with perfect alignment. The problems which are additive - incorrect stern tube boring and shaft alignment combine to produce an unscientific monster.
Points which should be a feature of the solution to the problem of the prior art are:
1. The bearing should not be subject to wear, or at least the wear should be minimal.
2. Lubrication, and hence security of the bearing, should be stable down to at least one r.p.m. and preferably at all speeds since very excessive wear can be expected to result on plummer blocks and stern tubes by the very slow revolutions given to the shaft (several minutes per revolution) during warming up and cooling down processes when there is no possibility of a hydrodynamic film being established on the surface of the conventional bearings. PA1 3. The bearing should be self-aligning to allow for variations in shaft alignment both initially and due to loading, pitching and yawing movements during operation of the vessel. This alteration in alignment can have disastrous effects on a fixed bearing. PA1 4. The bearing should not be subject to whirl, and radial run-out of the shaft should be an absolute minimum to ensure stability and long life to any seal fitted to the shaft. PA1 5. The friction should be minimal. PA1 6. The bearing should be designed for a rubbing speed of about 1,000 feet per minute which appears compatible with the 15" shaft of the destroyer type vessel which rotates at approximately 250 r.p.m., or the three foot diameter of super tankers rotating at approximately 100 r.p.m.
The present disclosure provides a solution to the problems enumerated above, and although the bearings here described are perforce of higher initial complexity than the simple wood bearings or oil or greased lubricated bearings employed in the past, their life is considerably longer and if not abused can be expected to equal or out-perform the life expectations of the main propulsion machinery in the vessel. Because the new bearing provides for a stable shaft, accurate seals of high tolerance can be employed and with subsequent longer life and absence of failure.